Kinda wanna watch/read something that not that much people know, idk why, just do.
Mermaid cannibalism cooking manga, cant go wrong.
Ningyohime no Gomen ne Gohan
essentially a big deep dive into the entertainment industry, from reality tv to idols, it's gonna cover a lot of shit
Oshi no Ko
Feng Shen Ji
try this out, i haven't read it (yet) but i heard its amazing
Space Brothers
Dennoh Coil
ODDTAXI
Full Metal Panic Fumoffu
Honey and Clover
For Fumoffu, I suggest watching at least several episodes of the first season beforehand to get a feel for the characters.
Its not really underrated but rather under watched, which is what I think you were looking for.
Planetes
A lot of people ask me about underrated shows. What I tell them is to just go back over the last 20 years and look at the highly rated shows for each season. More often than not you'll find shows that were highly rated but that you have never heard of. Usually this happens because it was overshadowed by another anime that debuted the same season or sometimes the season right after. Spring of 2013 saw the release of Attack on Titan. That season also had one of my favorite anime.
Hataraku Maousama
The season before had some great shows also.
Magi The Labyrinth of Magic
These shows are not under rated but they are definitely under recommended.
Mekakucity Actors
Kagerou Daze
Mekakucity Actors and Kagerou Daze are the same series, but you should experience both since they are like different timelines/endings. It's a complicated storyline.
IDOLiSH7
IDOLiSH7 is actually really popular in Japan, but not well known/promoted to Western audiences.
GOBLIN SLAYER
this is definitely not uncommon but hugely underrated